Considerable attention has been given to the use of hydrogen as a fuel or fuel supplement. While the world's oil reserves are rapidly being depleted, the supply of hydrogen remains virtually unlimited. Hydrogen, although presently more expensive than petroleum, is a relatively low cost fuel. Hydrogen has the highest density of energy per unit weight of any chemical fuel and is essentially non-polluting since the main by-product of burning hydrogen is water. Hydrogen is typically used as a chemical reducer in industrial processes or as a fuel to power fuel cells or hydrogen combustion engines, which in turn provide power to vehicles or other applications. To operate efficiently, such applications require a highly pure stream of hydrogen, free from impurities such as carbon dioxide or carbon monoxide. Contaminated hydrogen streams containing such impurities can greatly reduce the lifetime and operating efficiency of a fuel cell. Contaminated hydrogen streams may also dramatically affect the operating efficiency of hydrogen combustion engines resulting in shorter ranges for vehicle applications.
Hydrogen is also widely used as a reactant for chemical or food production. Examples of foods requiring hydrogen for processing are edible fats and oils. Typically unsaturated fats and oils are hydrogenated with hydrogen in the presence of a catalyst. Petroleum refineries use increasing quantities of hydrogen to meet regulatory requirements on diesel, gasoline, and other petroleum products. Petroleum refineries also use hydrogen to form gasoline from crude oil. Such reactions typically require highly pure sources of hydrogen to be effectively carried out. Pure hydrogen is also a necessity in the semiconductor industry where increasing line densities require hydrogen of the utmost purity. Impure hydrogen can be adverse to such reactions, especially in wide scale manufacturing processes, as the impurities can react to produce unwanted side products, which must be removed from the final product, or reduce the reaction efficiency, resulting in higher production costs. Several metallurgical operations are carried out in reducing atmospheres. Depending upon the application, they require different levels of hydrogen purity.
Typically, hydrogen is produced by a variety of methods such as water electrolysis, steam reforming, cracking hydrocarbons, or cracking ammonia. The methods using fossil fuels have a built in disadvantage in that the hydrogen streams produced by such methods contain contaminates such as carbon monoxide, carbon dioxide, methane, and various hydrocarbons. When producing hydrogen from fossil fuels, the hydrogen that is produced usually needs to be dried and cleaned of all contaminants before being used for many applications.
Different measures have been taken to remove various contaminants from hydrogen streams. Apart from adding a Stage 2 reformer, where the contaminants of Stage 1 are purified, there are other methods have been used to attempt to purify the hydrogen stream. To remove carbon monoxide, methods such as selective oxidation of CO in the presence of a catalyst are presently used, however, this results in a build-up of CO2 within the hydrogen stream. Selective adsorption of impurities in the impure hydrogen stream via Pressure Swing Adsorption to provide a pure hydrogen stream is also used, however, while this results in a product with a high purity, not all of the impurities are removed. Other methods used to remove contaminants utilize conventional metallic membranes or hydrogen permeable membranes to purify hydrogen, but purification is not 100%, so they may also require the use of a methanation catalyst, which catalyzes the conversion of CO to methane (CH4). While the presence of methane in the hydrogen stream may be suitable in some systems, other systems may be adversely affected by its presence. Hydrogen getter materials have also been used to purify hydrogen streams but they require operating temperatures above 250° C. and some of the contaminants still remain in the hydrogen stream after “purification”. Therefore, a system that can effectively purify an impure stream of hydrogen by removing contaminants contained in the impure stream will be highly desirable and have widespread uses for a variety of applications.